Well...I didn't actually buy Platinum. :x
But I have to say, if I did, it would've been worth every cent. Platinum was a major improvement over Diamond and Pearl. Firstly, the game speed increased. I'm not sure if anyone else noticed this, but while playing Pearl, there were always little pauses after every action in a battle. One pause in a battle wouldn't be such a big problem, but a lot of these pauses in the game would accumulate, making the battle drag on unecessarily. In Platinum, however, these pauses were removed, so the game was much faster than the previous games.
Secondly, there were more Pokemon available in Platinum. Diamond and Pearl had a lot of exclusive Pokemon that could only be caught in either game. However, in Platinum, many of these exclusive Pokemon were included in Platinum. Even though Platinum didn't include all of the exclusive Pokemon, the more significant ones such as Palkia and Dialga were really helpful. It was no longer necessary to possess both Diamond and Pearl to have Palkia and Dialga. Furthermore, in Platinum, these two legendaries are found at the higher level of lv70. Though Giratina was levelled down to lv47, the inclusion of Giratina's Origin Forme made up for this.
One major asset of Platinum was the inclusion of the Battle Frontier to the 4th generation games. From Platinum onwards, players were able to take part in the Battle Frontier until HeartGold and SoulSilver. This gave players something to do after the completion of the game. I myself found the Battle Frontier much more engaging than the Battle Tower in Diamond and Pearl.
There were also a lot of graphics improvements as well. In Diamond and Pearl, the graphics were hardly appealing at all, especially the battle scenes. In Platinum, the graphics of the battles was improved. Pokemon back sprites were improved so that the Pokemon mimicked their front sprites' animations.
In the end, I'd say that Platinum was definitely not a waste of money. It was a significant improvement over Diamond and Pearl, and improved gameplay, graphics and overall enjoyment greatly. Overall, Platinum was better, and even with the release of Pokemon Black and White, I'd reccommend people to buy Platinum, if only for enjoyment.